Here is a Technical Service Bulletin from Ford that pertains to your truck for repeat heater core leaks. At the end of the tsb it talks about installing a restictor in the heater hose. This restrictor will prevent excess pressure from going through the heater core. I would check the other items out that it talks about also.
CLIMATE CONTROL - REPEAT HEATER CORE FAILURE
LINCOLN:1985-1992 MARK VII1985-2002 CONTINENTAL, TOWN CAR1993-1998 MARK VIII2000-2002 GLS1998-2002 NAVIGATOR
MERCURY:1985-1994 TOPAZ1985-1997 COUGAR1985-2002 GRAND MARQUIS1986-2002 SABLE1991-1999 TRACER1995-2000 MYSTIQUE1999-2002 COUGAR1993-2002 VILLAGER1997-2001 MOUNTAINEER
Some vehicles may exhibit (repeat) heater core leaks. This may be caused by a chemical reaction called electrolysis. Electrolysis involves an ion exchange between the heater core and engine coolant which can result in a breakdown of the heater core material. This is similar to the operation of a battery.
Check for electrolysis on any vehicle with a heater core failure. If electrolysis is verified, flush the coolant and follow additional steps as required. Refer to the following Service Procedure for details.
If there is a condition of a heater core leaking or repeal heater core leak, check for electrolysis using the following procedure:
1. To check for electrolysis use a DVOM set on DC volts. Place the positive probe of the meter in the engine coolant and the negative probe on the negative battery post.
4. If there is still excessive voltage present in the coolant, check the engine to body/battery grounds. Also, verify proper grounding of any aftermarket electrical/electronic equipment which has been installed into the vehicle. Improperly grounded electrical devices can cause electrolysis to occur.
Coolant Fill Procedure
At times, in order to completely remove any trapped air in the cooling system of vehicles equipped with 4.6/5.4/6.8L modular engines, it may be necessary to use the following procedure:
1. Disconnect the heater hose at the right front or rear of the engine.